Microwave-Assisted Transesterification of Waste Cooking Oil Using Modified CaO Catalyst for High-Purity Biodiesel Production

Abstract

Biodiesel is a renewable alternative fuel capable of partially or fully replacing conventional diesel in compression ignition engines. Waste cooking oil (WCO) represents a sustainable feedstock for biodiesel production; however, its high free fatty acid (FFA) content must be reduced below 1% to prevent emulsion formation during methanolysis. In this study, WCO was pretreated and converted into biodiesel via a two-stage process (esterification followed by transesterification) using a microwave-assisted heating system and ammonium carbonate-modified CaO catalyst. The effects of reaction time (2–10 min) and catalyst loading (1–5 wt%) on biodiesel yield and quality were investigated. Under optimal conditions, a biodiesel yield of 82.80% was achieved at 4 wt% catalyst and 6 min reaction time. The resulting biodiesel exhibited a density of 0.8617 g/mL, kinematic viscosity of 2.863 mm²/s, and a calorific value of 36.78 kJ/g (8.81 kcal/g), all complying with SNI 7182:2015 standard. Gas chromatography-mass spectroscopy analysis confirmed the formation of high-purity fatty acid methyl esters, with a total methyl ester content of 97.71%, dominated by oleic, palmitic, linoleic, stearic, and ricinoleic methyl esters. These findings indicate that microwave-assisted transesterification with modified CaO catalyst is an efficient, rapid, and sustainable method for producing high-quality biodiesel from WCO, suitable for application as a renewable diesel alternative.